We have suggested mechanisms whereby the DNA double strand break (DSB) the purported biologically significant lesion (1) is produced intracellularly by ionizing radiation (2). The mechanisms predict that other locally multiply damaged sites (LMDS) are produced in which none strand break damage is present. These are predicted to be formed in greater yield than actual DSBs. The individual constituent damage sites of an LMDS can be produced, separated on the opposite strands of the DNA by relatively large distances on the molecular level, up to 10+ base pairs. Such well separated damage sites could be repaired as if they were singly damaged sites land would then not be hazardous to the cell even though they would assay as LMDS in the usual assays. In the proposed work, we aim to measure the yields of the various types of LMDS in model systems initially using SV 40 DNA in solution protected to the same OH' scavenging state as in the cell and then SV 40 in chromatin then within the mammalian cell. The response of the SV 40 DNA will then be compared to that of cellular DNA using the alpha tandem repeat sequence of the African green monkey as a model. We will then determine the distribution of separation of the constituent damages of LMDS on the opposite Strands of DNA, again initially with the SV 40 model system, then extending to the cellular system as described above. Finally after developing the assay of alpha DNA fully, we will explore the ability of cells to repair the various types of LMDS (non-strand break vs strand break, short separation vs long). Thus, we hope to define clearly the variety of significant DNA damage which must be considered during repair. This will aid in devising a specific assay for significant damage and in assigning the enzymatic processes by which they can be repaired.